The present invention is related to a thermal stabilizing system for a papermachine headbox, wherein the surfaces of the beams of the headbox which are in contact with the surrounding atmosphere are heated to the same temperature as the surfaces which are in contact with the stock material flow in a papermachine.
When the temperature of the stock varies, the beams of a headbox tend to deform due to temperature differences formed between the surfaces in contact with the stock.
Due to the deformations of the beams, changes occur in the stock cross-section of the flow. These changes are detrimental, at the outlet point of the stock in particular.
The flow rate of the stock flowing out from the headbox should remain constant across the entire width of the flow area within an accuracy of 1 to 2%.
The height of the flow outlet should remain constant in the same accuracy range. For example, if the slice opening is 10 mm, it should vary about 0.1 mm at the most across the entire width of the lip, which can be about 5 to 10 m.
It is known that attempts have been made to control the deformations of headbox beams by heating or cooling the surfaces which are in contact with air. The controlling is carried out by measuring the stock temperature or the beam deformation.
For example, Canada Pat. No. 849817 discloses a headbox wherein a liquid is heated to a temperature substantially equal to the stock temperature and is caused to flow in contact with the supporting frame of the headbox. The system comprises automatic control apparatuses for maintaining the temperature and the rate of flow of the liquid within preset limits.